The present invention concerns a microwave synthesizer with fractional division.
It applies notably to the production of synthesizers for radiocommunication systems with a high transmission rate, known as TDMA (Time division multiple access) and TACAN (Tactical Air Navigation).
Typically, TDMA synthesizers are synthesizers with a very short acquisition time of 15 .mu.s at a step of 3 MHz, which are used for the emission and reception of information in the 1242 to 1479 MHz frequency band. TACAN synthesizers, however, are synthesizers with a slow acquisition time of 1 ms at a step of 1 MHz, which are used for the emission of information in the 1025 to 1150 MHz frequency band and for the reception of information in the 1223 to 1474 MHz band.
In the known frequency synthesizers with fractional division, the rank of division is not fixed but varies between the ranks N and N+1 according to a law calculated in real time, which gives an elementary frequency step which is a submultiple of the reference frequency. This division is generally made by considering M reference periods, during which there are F divisions by N+1 and (M-F) divisions by N. In this way an average rank of division Nm is obtained such that: EQU Nm=(F(N+1)+(M-F)N)/M=N+F/M (1)
The synthesized frequency FS is then written as a function of the reference frequency FR: EQU FS=(N+F/M)FR=(NM+F)FR/M (2)
This principle of synthesis introduces a cyclic phase error in the synthesizer's phase comparator, provoking a frequency modulation of the voltage controlled oscillator, and the modulation frequency is equal to the fractional frequency shift. This modulation is usually eliminated by means of an active compensation device which corrects the phase error. This device is however not applicable to the TDMA application synthesizers, for although it strongly attenuates the fractioning frequency lines, it is not compatible with the acquisition time required. If the switching time remains acceptable for a small frequency jump, this is no longer the case for larger jumps. This is due essentially to the successive saturations which occur in the loop integrating amplifier during the servo phase.
This is why the known solutions used to make these synthesizers generally use techniques of direct or indirect frequency synthesis.
A known example of a direct frequency synthesis technique whose principle is based on the switching of 4 controlled oscillators, whose frequencies are repeatedly added and divided to obtain the step required, gives a switching time of the frequencies used much less than 15 .mu.s. However, this technique requires the use of costly and bulky components to make the frequency division and mixing circuits and the successive filters, and the many parasitic frequency lines generated by this type of device make it an unsuitable solution. For the same reasons the known principle of direct synthesis with selection and mixing of harmonics using SAW filters cannot be adopted either.
In the indirect synthesis technique a known principle of synthesis with two phase locking loops, using one "large step" and one "small step" loop with a pre-positioned oscillator and a transposition oscillator theoretically enables the acquisition time required to be obtained, but it gives a synthesizer which is too bulky and consumes too much for the applications envisaged.